In a still camera system that uses a single perforation per frame or other frame interval periodic film resident registering fiducial, information concerning the velocity and proximity of the film to the next available frame stopping point is needed if frame-to-frame transport time is to be minimized. Given this information, the motor speed or drive means can be controlled to provide the optimum drive for any given design.
Hosono U.S. Pat. No. 4,075,644 shows a camera system which uses a mechanical claw or pawl arrangement for stopping a film transport motor to bring the next available frame into registration with an exposure gate after exposure of the previous frame for a film having a single perforation per frame. In such a system, the drive means operates to move the film at constant speed until the motor is switched off when the pawl detects that the perforation has moved to the intended film stopping point. Registration is indicated when the pawl passes through the perforation into an alignment slot in the film feed wall as in Hosono or into a corresponding slot in a film cartridge as in Asano U.S. Pat. No. 4,141,635. Moving the film at constant speed limits the ability to achieve high speed frame-to-frame transfer. There is a tendency when the film is moved at high speeds for the frame to overshoot the exposure station, so that when the pawl forces registration by mechanical means, the forces acting to continue the movement may cause image distortion or emulsion damage due to stretching or stressing the film.
It is known in single speed still camera film drive mechanisms to prevent overshooting stresses by switching the motor off at a point prior to the stopping point, so that the film will coast into the alignment position due to the built-up momentum. The position of the film at motor switch-off is estimated based on elapsed drive time since motor actuation. The relationship between actual position and elapsed time, as well as the coasting speed and conditions, is, however, dependent on gear wear, battery temperature and other factors which may vary in unpredictable ways.
It is known in connection with cameras utilizing unperforated film as in Studley U.S. Pat. No. 3,677,159 and for single lens reflex cameras using multiple sprocket engaging perforations per frame as in Takahashi U.S. Pat. No. 4,431,292 to start reducing film transport velocity before the stop point. Most such systems track the drive means, not the progress of the advancing film itself to determine how frame-to-frame movement is progressing. In Takahashi, the travel distance of the film is tracked by counting the number of perforations which pass a given point, viz. the location of oppositely disposed LED and photodiode elements. For a 35 mm filmstrip having eight equally-spaced sprocket perforations per frame, Takahashi operates the wind-up motor at full speed until a count of six perforations is reached, and then reduces the running speed by either temporarily interrupting or limiting power to the motor for the remaining count of two perforations. Takahashi neither tracks the position of the film between perforations, nor monitors the speed of advance of the film. Such an arrangement is, therefore, not useful with films of the type having a single perforation per frame, or for providing film transport velocity and positional information to provide feedback for motor control during intervals of movement between perforations.
Other film transport speed deceleration systems are shown in Iwashita et al. U.S. Pat. No. 4,175,845; Shiozawa et al. U.S. Pat. No. 4,474,442; and Kawamura et al. U.S. Pat. No. 4,697,899. Such arrangements do not receive feedback through monitoring film resident fiducials.